Analog-to-digital converters (A/D converters) are well known in the art as circuits that change a continuously varying voltage or current into numerical digital form. The input may be AC (alternating current) or DC (direct current), and the output may be serial or parallel, binary or decimal. A/D converters produce a digital output code that is a function of the analog input voltage and a voltage reference input. Moreover, the width of a given digital output code corresponds to a range of analog input voltages for which that code is produced.
In an ideal A/D converter, each output code has an identical width, meaning the range of analog input voltages remains constant from one code transition point to the next. However, practically speaking, all A/D converters suffer from non-linearity due to their physical imperfections, causing their outputs to deviate from an ideal linear function.
There are two measurements of non-linearity: differential non-linearity (DNL) and integral non-linearity (INL). DNL occurs when the range of analog input voltages is not uniform for all digital output values. As such, DNL indicates the difference between the actual output code width and the ideal code width of one least significant bit (LSB). DNL may be caused by inaccuracies in capacitor sizing that often leads to missing codes in the output of the A/D converter. Missing codes in the output results in granularity and effectively reduces a dynamic range of the converter's output. INL is the cumulative deviation over a number of consecutive code values, i.e., cumulative DNL errors, and specifies how much the overall transfer function deviates from an ideal linear response.
Conventional methods for determining the DNL and INL of an A/D converter include using either a quasi-DC voltage ramp or a low-frequency analog voltage as the input. A simple DC-voltage ramp test can incorporate a logic analyzer, a high-accuracy digital-to-analog converter (D/A converter), and a high-precision DC source for sweeping the input range of the A/D converter being tested. If the setup includes a high-accuracy D/A converter, the logic analyzer monitors offset and gain errors by directly processing the A/D converter's output data. The precision signal source creates test voltages for the converter being tested by sweeping through the input range of the converter from zero to full scale. Once reconstructed by the D/A converter, each test voltage at the A/D converter input is subtracted from its corresponding DC level at the output of the D/A converter, producing a small voltage difference that can be displayed with an X-Y plotter and linked to the INL and DNL errors. A change in quantization level indicates differential nonlinearity, and a deviation of the differential voltage from zero indicates the presence of integral nonlinearity.